In LTE-A (Long Term Evolution-Advanced), Carrier Aggregation (CA) is introduced as a technology capable of transmitting a larger volume of data. In CA, a mobile station (US) and a base station (eNB) perform data transmission and reception using multiple LTE wireless carriers called Component Carriers (CCs).
When the data transmission and reception is performed by CA, the mobile station is assigned a frequency resource of good radio quality with respect to each CC by frequency scheduling. Therefore, in CA, an amount of frequency resource assigned may differ among the CCs.
Furthermore, in CA, independent TPC (Transmission Power Control) with respect to each CC is possible. Therefore, a transmission power value and PSD (Power Spectral Density) of each CC may differ among the CCs.
Non Patent Document 1: 3GPP TB36.814, “Further advancements for E-UTRA physical layer aspects (Release9)”, V9.0.0 (March 2010)
However, a conventional wireless communication system into which the above-described CA is introduced has the following problems.
In the conventional wireless communication system, a mobile station calculates a transmission power value of each CC on the basis of a TPC command transmitted from a base station. Then, the mobile station converts a difference in transmission power value between CCs into an amplitude ratio, and adjusts the amplitude of each CC on the basis of the amplitude ratio. Therefore, for example, when there is a large difference in transmission power value between CCs and the mobile station performs the amplitude adjustment based on a CC having she maximum transmission power value, an amplitude level of a signal of a CC having a small transmission power value is lowered, and a quantization error is increased. As a result, there is a problem that the signal quality of a CC having a small transmission power value is degraded.
On the other hand, when the mobile station performs the amplitude adjustment based on a CC having the minimum transmission power value to avoid degradation of the signal quality of a CC having a small transmission power value, an amplitude level of a signal of a CC having a large transmission power value is increased, and overflow occurs in a on converter. Namely, there is a problem that this overflow may cause degradation of the signal quality and unwanted out-of-band signal interference.
Furthermore, measures against the overflow could include a measure to increase a quantization bit rate; however, when this measure is implemented, there are problems that the circuit scale of the DA converter is increased, and further, power consumption is increased due to the increase in the circuit scale.